1. Field of the Invention
This invention relates broadly to turbine jet (turbojet) engines. More particularly, this invention relates to afterburners that increase thrust and facilitate the starting of a turbojet engine.
2. State of the Art
Most small disposable missiles in use today are propelled by solid fuel rocket engines as opposed to liquid fuel injected turbojet engines. The selection of an engine powered by solid fuel as a propulsion device for small disposable rockets and missiles has been largely dictated by three factors. First, solid fuel rocket engines are simpler to design and more affordable to construct than liquid fueled turbojet engines. Second, solid fuel propellant is a relatively high density high energy fuel source which burns hotter than liquid fuel. Because of its high density high energy characteristics, a smaller quantity of solid fuel is required to produce an equivalent amount of thrust. Further, solid fuel is easier to install within an engine and remains relatively stable until ignited. Once ignited, solid fuel rocket engines produce an intense blast or xe2x80x9cburstxe2x80x9d of thrust and continue to produce thrust at a substantially constant rate until all the solid fuel is depleted.
Unfortunately, solid fuel rocket engines have several important disadvantages. First, there is no way to stop the burn once the fuel is ignited. Second, there is little way to control the burn, augment the burn, or correct resulting inefficiencies caused by an uneven burn once the fuel charge is ignited. What small degree of thrust modulation or augmentation capability is available in such systems is typically provided either by introduction of an oxidizer into the combustion environment or an additive into the solid fuel itself. However, these solutions introduce additional complications and disadvantages to the already existing problems of the system.
Turbojet engines offer many advantages over the solid fuel rocket type engines typically used as propulsion systems for short ranges disposable rockets and missiles. Turbojet engines typically use readily available, relatively inexpensive liquid fuel which produces a relatively efficient burn for known temperature and air density parameters. Turbojet engines offer a considerably greater degree of control or modulation of thrust and therefore a greater degree of control of the flight path or trajectory of the missile.
While turbojet engines provide advantages over solid fuel rocket engines, there are several disadvantages. One important disadvantage of current turbojet engines, particularly for use in small disposable rockets and missiles, is that they are complicated and difficult to start. Turbojet engines require a starting sequence involving spooling up or prestarting the compressor to initiate combustion (i.e., to spool up the compressor to provide a sufficient mass flow of air and temperature through the combustion chamber), and using an auxiliary means, such as an explosive starter cartridge system to initiate combustion in the combustion chamber. Achieving a level of combustion necessary to result in a sustained burn is particularly difficult at high altitudes, e.g., over 30,000 feet, due to the relatively low levels of oxygen in the air at those elevations. However, aircraft-launched rockets and missiles may require starting at such altitudes.
Another disadvantage is that a greater quantity of liquid fuel is required to produce the same propulsive thrust as a smaller quantity of the denser solid fuel making the vehicle considerably heavier than it would otherwise be and requiring a greater space within the projectile to house fuel. Turbojet engines can be equipped with afterburners to produce a significant increased thrust component for a short period of time. Afterburners in jet turbines typically operate by injecting raw liquid fuel (i.e., spraying atomized fuel particles) into the stream of hot exhaust gasses venting from the combustion chamber of the engine into the tail pipe or aft nozzle of the engines. The atomized liquid fuel combines with the oxygen naturally present in the superheated exhaust as well as with unburned fuel in the exhaust remaining from combustion to create a xe2x80x9cburstxe2x80x9d of thrust for short periods of time. The xe2x80x9cburstxe2x80x9d of thrust simulates for a time the thrust produced by a solid fuel rocket engine. Afterburners use the already heated high velocity exhaust gases to increase the thrust thereby increasing the burn temperature and therefore the efficiency of the burn. Unfortunately, conventional liquid fuel afterburner injection systems are expensive to design and manufacture (particularly for disposable systems), are functionally complex, add considerable weight to an already weight sensitive system by increasing the liquid fuel requirement, are subject to clog or malfunction, require a separate or cooperating fuel supply and distribution system, and consume additional space within the already cramped confines of the vehicle. Further, such systems require use of special high cost materials able to withstand the extremely high temperatures generated in the aft nozzle portion of the engine. More importantly, because of the fuel burn characteristics of liquid fuels, the thrust generated by liquid fueled afterburners is not equivalent to the thrust produced in a solid fuel rocket engine.
For small engines like those used in small, disposable rockets or missiles, any additional weight or space requirement, functional complication, or expense decreases suitability for use. In order to replace solid rockets with turbojet engines as propulsion sources for disposable missiles, it is necessary to reduce the cost and weight, facilitate manufacture, and provide improved systems to start the engine and increase the short-term thrust produced by the engine.
It is therefore an object of the invention to provide a turbojet engine with an afterburner for use on small disposable rockets and missiles which is simple to design, affordable to construct, does not add substantial weight, is sized to fit within a space allocated for the propulsion system, and is efficient to operate.
It is another object of the invention to provide an improved afterburner for a turbojet engine for use on small disposable rockets and missiles which uses solid fuel to provide a boost of thrust for a period of time.
It is a further object of the invention to provide an afterburner system for a turbojet engine for use on small disposable rockets and missiles which facilitates starting the engine.
It is an additional object of the invention to provide a solid fuel afterburner for a turbojet engine which includes an oxidizer within the fuel to control the rate and temperature of the burn and which prevents the fuel from fully burning until the engine compressor has spooled up sufficiently to provide a required amount of oxygen to the exhaust gases directed to the afterburner.
In accord with these objects, which will be discussed in detail below, a turbojet engine is provided which includes a housing having a forward inlet nozzle, an aft exhaust nozzle, and a combustion chamber therebetween. The aft exhaust nozzle is preferably a variable area nozzle which permits optimization of engine performance. The engine also includes a compressor between the inlet nozzle and the combustion chamber, and a turbine between the combustion chamber and the exhaust nozzle. A shaft couples the compressor and the turbine together such that rotation of the turbine causes rotation of the compressor. A liquid fuel injector and atomizer are provided to inject atomized fuel into the chamber for combustion therein. In addition, a solid fuel element is provided in the aft exhaust nozzle, e.g., lining the inner surface of the exhaust nozzle.
The solid fuel may be used to assist the starting of the engine, particularly useful under adverse starting conditions. That is, when the compressor begins to spin up to pump air through the engine, the solid fuel in the exhaust pipe is ignited by an igniter which begins to combust with the oxygen rich air introduced by the compressor. This combustion increases the temperature in the entire engine, such that when liquid fuel is injected in the combustion chamber, the temperature is sufficiently high to allow the liquid fuel to burn in a self-sustaining manner. Thus, the engine is ignited from the back, and after ignition, the solid fuel continues to burn, providing an afterburner effect boosting the thrust until the solid fuel is depleted.
Alternatively, the solid fuel can be ignited after the liquid fuel is undergoing a self-sustained burn. In this situation, the solid fuel solely functions to provide an afterburner effect.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.